Conventional manufacturing processes for producing commercially available high-efficiency and large-area Si solar cells depend heavily on the use of thick crystalline Si wafer (100 μm˜500 μm). Due to the rigid and brittle nature of such thick crystalline Si wafers including both mono- and multi-crystalline Si wafers, wafer-based crystalline Si solar cells (100 μm˜300 μm) are not compatible with ultra-thin and fully flexible form factors suitable for covering curved surfaces. In addition, most solar modules and panels consisting of thick crystalline Si solar cells require heavy front glass covers and aluminum frames to protect them from environmental factors such as hail, rocks, wind etc., making them difficult to integrate into light-weight packaging using non-glass based materials such as fluorine-based plastics, carbon fiber, fiberglass or foam molding that can enable tailored output optimized to power systems.
Flexible crystalline Si solar cells can be manufactured using intrinsically ultra-thin crystalline Si wafers that are wire-cut into less than approximately 50 μm during wafer slicing of semiconductor ingot. However, overcoming the yield losses and handling issues such as cracks and breakage that are a problem for traditional thin wafer processing is extremely difficult and limits their usefulness in fabricating operationally suitable solar cells.
Conventional manufacturing methods for producing crystalline thin Si solar cells are based on epitaxial growth of monocrystalline Si layers onto a donor Si wafer followed by an epitaxial layer transfer process (LTP). For monocrystalline thin Si solar cells (less than 50 um), the epitaxial growth and LTP approach has demonstrated a potentially viable way to achieve high-performance crystalline thin Si solar cells. See R. Brendel, “Review of Layer Transfer Processes for Crystalline Thin-Film Silicon Solar Cells,” Japanese Journal of Applied Physics, Vol. 40 pp. 4431-4439, 2001. This approach uses a special surface conditioning of the donor Si substrate that permits the transfer of the active device layer from a re-usable growth substrate to a device carrier. A special surface conditioning method includes oxidation; implantation of hydrogen (H+) ions; and formation of porous Si, textured porous Si, and selective etching layers. However, this approach requires deposition or epitaxial growth of crystalline semiconductor layers using high-vacuum epitaxial tools (e.g., chemical vapor deposition (CVD)-based systems, molecular-beam epitaxy (MBE) systems, or high vacuum electron-beam evaporation systems) for device layer formation on a foreign or native substrate at high temperature, which requires extremely high capital expenditure (CAPEX) and high maintenance cost. It also requires optional re-crystallization in a high temperature (e.g. >700° C.) and layer separation processing including delicate cleaving or complex wet chemical etching.
Manufacture of ultra-thin flexible crystalline GaAs solar cells can also be enabled by epitaxial growth of GaAs layers on GaAs substrate and then epitaxial liftoff (ELO) process, which is epitaxial growth of a thin lattice-matched AlAs layer grown between the wafer and an active GaAs device, and then slowly selectively etched by HF acid. However, this approach also requires epitaxial growth of crystalline GaAs device layers using high-vacuum epitaxial tools, such as metal-organic CVD (MOCVD), MBE or hydride vapor phase epitaxy (HVPE) for device layer formation on a donor wafer. In addition, the ELO process is extremely slow and it suffers from poor post ELO wafer surface quality, requiring an additional surface treatment for wafer re-use.